In a wireless local area network (WLAN) according to the IEEE 802.11 standard, an access point (AP) in a cell coordinates packet transmission for all stations associated with the cell. A single wireless channel, i.e., frequency band, is shared by both the uplink from the station to the AP, and the downlink from the AP to the station for all data and control signals. Every station can communicate with the AP, whereas it is not required for any two stations to be within communication range of each other.
The transmission rate of the wireless channel can vary, depending on a perceived signal-to-noise ratio (SNR). For example, the physical layer of the IEEE 802.11b standard supports four rates at 1 Mbps, 2 Mbps, 5.5 Mbps and 11 Mbps.
IEEE 802.11e HCCA
To support a given quality of service (QoS), the IEEE 802.11e standard defines two operating modes: enhanced distributed channel access (EDCA), and hybrid coordinated channel access (HCCA). The EDCA mode is based upon carrier sensing multiple access with collision avoidance (CSMA/CA). CSMA/CA provides prioritized channel access for up to four access categories (ACs). Each AC is associated with a set of QoS parameters for channel contention, such as backoff values, to realize different services among the ACs.
The HCCA mode allows a hybrid coordinator (HC) located at the AP to poll stations for contention-free access during a contention-free period (CFP), and allocates a transmission opportunity (TXOP) at any time during a contention period (CP). During the transmission opportunity, the station can send one packet. HCCA enables parameterized QoS for each data stream. The HC allocates a transmission opportunity (TXOP) in both the CFP and the CP. Each TXOP specifies a start time and a duration of a transmission for a particular station. The traffic profile and QoS requirements of each data stream can be taken into consideration when centralized scheduling is used for TXOP allocation.
To regulate uplink transmission, the HC sends CF-Poll messages to each station in order to collect current traffic information, such as data arrival rate, and data size. The standard specifies a simple round-robin scheduling algorithm to poll each station during predefined service intervals according to a QoS contract.
Dynamic TDMA Based Scheme
Dynamic time division multiple access (TDMA) offers an alternative technique for parameterized QoS. The entire channel is divided into time slots, and multiple time slots form a superframe. The time slot allocation is performed by the AP, which takes into account the QoS requirements of each data stream. After the slots are allocated, all transmissions begin at the predefined time and last for predefined maximum durations at a granularity of a time slot. Thus, the signaling procedure and resource allocation is simply based on the channel access protocol.
The slot allocation is also adjusted regularly in order to accommodate short-term rate variations of applications. The AP can use several acknowledgement (ACK) policies, e.g., immediate ACK, delayed ACK, repetition, etc., to acknowledge reception of each packet. These ACK policies accommodate diverse applications and traffic types, e.g., unicast, multicast and broadcast transmissions. Furthermore, access slots, which are typically much smaller than data slots, are used by joining stations to send the AP requests such as association, authentication, resource reservations, etc. These access slots are typically contended for using via CSMA/CA or slotted Aloha.
The MAC design in the HiperLAN/2 (H/2) and the IEEE 802.15.3 standards adopts this dynamic TDMA based scheme to coordinate QoS-oriented channel access among contending stations.
Both the polling-based method and the dynamic TDMA-based method have drawbacks with respect to providing QoS in wireless LANs. The polling-based channel access method grants applications with QoS in a relatively flexible way. That method can handle variable packet size, and can accommodate short-term rate variations. However, this flexibility is achieved at the cost of high signaling overhead. The polling procedure incurs non-negligible channel inefficiency because every uplink data packet involves a polling message exchange with HC. Moreover, the polling messages are transmitted at the base rate, e.g., 1 Mbps according to the 802.11b standard, to accommodate different transmission rates of various stations. This further deteriorates the throughput.
The dynamic TDMA-based channel access method can efficiently provide QoS support for constant-bit-rate (CBR) multimedia applications, but not for variable-bit-rate (VBR) applications. Typically, the VBR applications, such as video-conferencing, have variable packet sizes, or time-varying source rates. Moreover, the TDMA-based method requires strict, fine-grained time synchronization at a ‘mini-slot’ level.
Another channel access method uses a “central coordination and distributed access,” Lo et al. “An Efficient Multipolling Mechanism for IEEE 802.11 Wireless LANs,” IEEE Transactions on Computers, Vol. 52, No. 6, June 2003. However, that method has several limitations. First, that method does not have a mechanism to accommodate the multi-rate physical-layer capability specified by the current IEEE 802.11 standard. Therefore, potential throughput gain is greatly compromised. Second, that method does not have a mechanism to accommodate short-term traffic variations while ensuring long-term bandwidth for each data stream according to its QoS contract. Third, that method does not have any policing mechanism to detect and penalize aggressive or misbehaving data streams that violate their QoS specifications.
To overcome the problems associated with the above channel access methods, U.S. patent application Ser. No. 10/888,398, “Sequential Coordinated Channel Access in Wireless Networks,” filed on Jul. 9, 2004, by Yuan et al., provides for a sequential coordinated channel access (SCCA) method. In the SCCA method, each station obtains a reserved transmission slot from the AP. Various scheduling algorithm can be applied to achieve an optimal system performance.
The SCCA method provides a highly efficient coordinated channel access. However, that method does provide the details of a high performance signaling method that can take advantage of the SCCA.
In the prior art random access method, resource reservation and allocation is not a concern, because there is no need to reserve channel access in advance. Each station that needs to transmit contends for the channel in a distributed manner. The IEEE 802.11e standard defines messages, such as ADDTS request, ADDTS response and DELTS, to facilitate the HCCA operation. However, the procedure and the corresponding frame format specified therein are not applicable for SCCA due to the special nature of SCCA scheme. Moreover, the signaling for HCCA mode is not efficient and considered undesirable for a high throughput network designed according to the IEEE 802.11n standard. Therefore, there is a need to provide a new signaling method, which can fulfill the signaling need of SCCA in a highly efficient fashion, offer great flexibility and extensibility while maintaining backward compatibility, and entail minimal additional implementation costs.